1. Field of the Invention
This invention relates to a method of orienting microphase-separated domains of block copolymers with respect to a substrate, and specifically to a composition and method of using the composition to orient microphase-separated domains of block copolymers with respect to a substrate.
2. Description of Background
Block copolymers films can be used as a part of an assembly system, in which nanoscale features form when blocks of the block copolymers phase separate into microdomains (also referred to herein as “microphase-separated domains” or “domains”) to reduce the total free energy. Such thin films of block copolymers provide features having spatial chemical contrast at nanometer-scale, and consequently by their ability to generate these periodic nanoscale structures have been used as a low-cost material for nanopatterning. One problem in block copolymer patterning is with respect to controlling the orientation of the assembled microdomains. For example, lamellae forming block copolymers (FIG. 1A) can align their domains parallel to the plane of the substrate on which they are coated (FIG. 1B), or perpendicularly to the substrate surface (FIG. 1C). When lamellae form parallel to the plane of the substrate, one lamellar phase forms a first layer at the surface of the substrate (in the x-y plane of the substrate), and another lamellar phase forms a second, parallel layer on the first layer, so that no patterns of microdomains are obtained when viewing the film along the perpendicular (z) axis; however, when lamellae form perpendicular to the surface, the perpendicularly oriented lamellae provide nanoscale line patterns. Without external orientation control, thin films of block copolymers tend to organize into randomly oriented nanostructures or undesired morphologies, which are of no use for nanopatterning because of the random nature of the features.
Orientation of block copolymer microdomains can be obtained by pairing the assembly process of the layer of block copolymer, with an external orientation biasing method such as use of a mechanical flow field, electric field, temperature gradient, or by the influence of surface interaction on the block copolymer by a surface modification layer. Of these, use of a surface modification layer for orientation control is relatively straightforward to integrate into a spin-casting or other film-forming manufacturing process, and is therefore desirable. Random copolymer brushes, thermally cross-linked random copolymers, and assembled monolayers have each been used as the basis of an orientation control layer to induce preferential orientation in block copolymer thin films.
An orientation control layer can present a neutral or a non-neutral surface to block copolymers. The orientation control layer can have a surface that is preferentially wetted by a particular block of the block copolymer; such a surface is considered not to be neutral. The surface of neutral orientation control layer is wettable by more than one block in the block copolymer. Therefore, on a neutral orientation control layer, the cylinder-forming and lamellae-forming block copolymer form laterally microphase-separate domains which orient perpendicularly to the neutral orientation control layer. Typical neutral orientation control layers have been prepared by casting a film of a random copolymer comprising the monomers of each block. For example, a neutral orientation control layer for the poly(styrene-b-methyl methacrylate) diblock copolymer can be made from a random copolymer of styrene and methylmethacrylate.
While surface modification methods, particularly use of orientation control layers, can be integrated into the manufacturing process, additional processing steps are necessary, including dispense, spin, bake, or rinsing steps, to apply the orientation control layer prior to casting block copolymer thin films, the sum of which extend process cycle time and increases the complexity of the processing sequence. Longer processes (i.e., those having more processing steps) are not generally desirable, as such processes introduce more opportunity for introduction of coating defects, reducing overall device yield and increasing the cycle time for producing devices.